In the field of biological process engineering, different commercial sensors or electrodes are currently available for detecting mainly physicochemical parameters such as temperature, pH, redox potential, dissolved oxygen and some ions. These elements record data, and most thereof control the parameters during one of the individual stages of which a specific biotechnological process is comprised. Thanks to this type of elements, it is possible, for example, in different types of industrial fermentation, to take measurements reflecting the status of any certain component during the microorganism culture process, which is especially important for scientists and operators in order to perform proper in-process control.
In most fermenting and in some separation processes, a proper control of the process dynamics is required, for which purpose estimating and determining the evolution of the concentration of a certain component contained in the fermenting medium is especially important. By means of a simple measurement, it must be possible to determine or estimate the amount of catalyst or by-product present in the culture medium. The concentration of the component in question may be modified in a controlled manner provided that it be possible to instantaneously detect the element in question and, therefore, that the proper detectors, in other words, transducers affording the possibility of converting the concentration of a certain parameter into an electrical signal, be available.
Different procedures for determining components in the fermenting medium are current state-of-the art, although the main microbiological problem which has become more acute over the course of time lies in this type of procedures being highly costly in terms of the time and material involved.
Conventionally, the measurement of the concentration of the substances present in the culture medium is made using different analytical techniques (dry weight measurement, spectroscopy, chromatography, etc.) on a sample of the medium taken from the bioreactor. This technique entails the drawback of a delay in obtaining the results (which can be significant in relation to the growth dynamics of the microorganism) and the small number of samples during the fermenting process.
The techniques utilized for the biomass concentration measurement are as follows:                Dry weight measurement: The liquid medium is separated from the solid (which is comprised mainly of microorganisms), the dry extract then being weighed. This is the most reliable and accurate measurement of the quantity of biomass per unit of volume, but it entails the drawback of not being possible to measure continuously and of the lag in the obtaining of the data.        Measurement of the optical density or absorbance to a wavelength. According to Beer's law, the absorption of monochromatic light by a medium is proportional to the concentration of substances which absorb the energy at that wavelength suspended in a medium transparent to that radiation, which affords the possibility of estimating the concentration of cellular microorganisms or other substances.A=log(Po/P)=ε*b*cWhere:            A: Absorbance of the sample    Po: Intensity of the light source    P: Intensity of the beam after passing through the sample    ε: Specific absortivity    b: Thickness of the sample    c: Concentration of the absorbing substance in the sample
This method can be used for the on-line measurement of the concentration by using a probe inserted into the bioreactor or by means of the continuous recirculation of the medium through the measuring device. Beer's law is only valid for low biomass concentrations. However, a relationship does exist (although non-linear) between the biomass concentration and the absorbance to media and high concentrations. In order for the measurement to be useful, there must be no significant absorption of other substances in the medium to the wavelength selected for making the measurement. Air bubbles or other solids present in the medium may interfere with the measurement.                Measurement of the stirring power: Some microorganisms (such as fungi) modify the apparent viscosity of the medium in terms of the total biomass present therein. This phenomenon may be utilized for making an estimate of the quantity of biomass in the medium by means of the measurement of the intensity consumed by the motor that moves the stirring blades and the angular speed thereof.        Measurement of the capacitance of the medium: When subjected to an electrical field, microorganisms may act as dipoles. If the culture medium is used as a dielectric between the plates of two electrodes and a sine current (0.1-1 MHz) is applied, the resulting capacitance is a function of the concentration of viable cells existing between the plates of the electrodes. This method is used in different sensors available on the market.        
Research has been conducted related to documents of patents having to do with the present invention, such as:                JP-63015140 Turbidity sensor        U.S. Pat. No. 5,446,544 Turbidimeter        EP-0,590,487 Device for turbidity measurement in aqueous media        U.S. Pat. No. 5,828,458 Turbidity sensor        EP-0,869,350 Turbidity measuring system        U.S. Pat. No. 3,962,041 Method and apparatus for measuring the opacity of fluids        U.S. Pat. No. 4,893,935 Apparatus and method for optical density measurements of biomass processes        U.S. Pat. No. 3,714,445 apparatus for optical measurements of microbial cultures        U.S. Pat. No. 3,727,066 Probe photometer with fluid sensing device        U.S. Pat. No. 3,819,278 Turbidity measuring device with means for preventing the formation of bubbles        
After analyzing these documents, it is our understanding that none thereof foregoes the present invention.
The measurement of the biomass and/or of some of the products is important, given that it affords the possibility of calculating the mass balances in the method, is necessary as a point of reference for calculating the specific rates at which the substrate is consumed and the product is generated, is an indicator of the kinetic evolution of the developing cells, in addition to being decisive for control purpose. For the purpose of achieving this objective, some sensors and methods have been devised over recent decades for estimating the biomass concentration in the reactor. Most of them make the estimate of the biomass through the use of optical principles, although there are estimating methods based on the dielectric properties of the biomass and on the changes in density of the cultured media, as has been discussed at an earlier point hereinabove.